Simplify the following expression: $a = \dfrac{-5z}{8} - \dfrac{-9z}{8}$
Answer: Since the expressions have the same denominator we simply combine the numerators: $a = \dfrac{-5z - (-9z)}{8}$ Combine like terms: $a = \dfrac{4z}{8}$ Simplify by dividing numerator and denominator by 4: $a = \dfrac{z}{2}$